Just as I suspected, the server is denying the mount due to
(I'm guessing) your exports.
Fire up wiresharke and read in the data file you posted.
Then use 'rpc.program == 100005' as a display filter
and you will see for your self, that the server is
returning an "ERR_ACCESS"

hmm... this is a strange one... On elwood (the rawhide machine)
can you post a network trace of that failure? It may
shed some light on whats going on....
In general, a "Unknown Host" means its a DNS problem but
I'm assuming other apps are not having the same problem..
Question: is IPv6 enabled on any of your network interfaces?

We've just run into this problem on two of our RHEL5.5 systems. That is, systems that had previously been exporting NFS filesystems without a problem suddenly started reporting "permission denied" despite entries like this in /etc/exports:
/mnt/somefilesystem *(ro)
The solution was to manually mount /proc/fs/nfsd.
/etc/modprobe.d/modprobe.conf.dist is unmodified from the stock configuration and includes the necessary install/remove rules:
install nfsd /sbin/modprobe --first-time --ignore-install nfsd && { /bin/mount -t nfsd nfsd /proc/fs/nfsd > /dev/null 2>&1 || :; }
remove nfsd { /bin/umount /proc/fs/nfsd > /dev/null 2>&1 || :; } ; /sbin/modprobe -r --first-time --ignore-remove nfsd
This happened on unrelated systems, maintained by two distinct groups of people.
Since the mount operation is idempotent (if the filesystem is already mounted, running "mount -t nfsd nfsd /proc/fs/nfsd" simply reports an error), we've simply made this part of the NFS startup script. It seems that this might be a good idea in general.